Cells, recovered from afferent and efferent lymph of sheep popliteal nodes responding to Concanavalin A,E. coli lipopolysaccharide, dinitrophenylated bovine serum albumin, allogeneic and xenogeneic cells, will be examined for the production of factors which enhance or suppress in vitro and in vivo reactivity of cells responding to autologous and heterologous antigens. (1) Supernatant fluids, prepared from cells collected during defined intervals of the in vivo response will be tested for modulatory activity on (a) blastogenesis of lymphocytes to mitogen, allogeneic and xenogeneic cells by examining DNA and protein synthesis and morphological changes (b) immunoglobulin synthesis by lymphoblasts collected from the efferent lymph using plaque assays and 3H-leucine incorporation into secreted protein. (2) Active supernatants will be infused into the node to determine their effects on the induction and establishment of an immune reponses; hence in vitro activity will be correlated with in vivo enhancement or suppression. (3) The active factors will be isolated and characterized biochemically. (4) In responses to allogeneic and xenogeneic cells, the cells in the efferent lymph will be constantly monitored for responsiveness to the stimulating antigen throughout the response. The results of these experiments, along with those of sections (1) and (2) will distinguish between an active suppressive mechanism (specific or non-specific) and a depletion of antigen-reactive cells by the stimulated node to account for the observed non-reactivity of such cells at defined intervals during the immune response. (5) The cells responsible for the production of the active factors will be separated into subpopulations to define the contribution of polymorphonuclear leukocytes, monocytes, T and B lymphocytes and histamine receptor bearing cells to the regulatory mechanisms.